The present invention relates to a fuel control apparatus for an internal combustion engine. More particularly, it relates to an improved system for burning at a high temperature a deposit on the surface of a hot-wire type intake air quantity sensor used for the fuel control apparatus.
The characteristics of hot-wire type intake air quantity sensors change depending on substances deposited on the surface of the sensors. Deposition of the substances causes an error in the quantity of fuel to be supplied to an engine, whereby there arise problems of deterioration of the exhaust gas and reduction in operational performance. To cope with the problems, the hot wire has been heretofore heated at a temperature higher than a temperature in the normal operation of the engine after the engine has been stopped to thereby burn off the deposit. A method of burning-off of the deposit is disclosed, for instance, in Japanese Unexamined patent publication No. 76182/1979.
It is revealed from experiments that temperature for heating the hot wire should be about 1000 .degree. C. in order to obtain an effective burning-off operation. However, when the hot wire is heated at 1000.degree. C., a fuel-air mixture is possibly fired. The firing of the fuel-air mixture is inconvenient for the operation of the intake air quantity sensor disposed in an intake passage for a gasoline engine.
Heretofore, in order to avoid the firing of the fuel-air mixture, the burning-off operations have been carried out only when the temperature of the intake passage and the revolution of the engine satisfy predetermined conditions during its operation and the fuel-air mixture exessively supplied to the intake passage during warming-up operations for the engine is sufficiently scavenged.
Alternatively, the burning-off operations have been carried out before the fuel-air mixture reversely flowing from a fuel supplying portion reaches the intake air quantity sensor and after the engine has been stopped. The reversing time of the mixture is obtained by experiments.
However, even when the above-mentioned conditions are satisfied, there sometimes remains gasoline around the intake air quantity sensor, and the firing of gasoline has happened due to a reaction in the burning-off. Thus, it has been found that the burning-off condition couldn't provide a good result.
Further, an erroneous operation of an electronic control unit or the intake air quantity sensor which results from a reduction of voltage in a circuit system including the electronic control unit and the sensor sometimes causes the burning-off while gosoline is supplied to thereby fire the gasoline.